This invention relates generally to the field of camera electronic displays and in particular to a drive circuit for electroluminescent (EL) backlights used with liquid crystal display (LCD) camera displays.
LCD displays are commonly used on cameras to provide film and camera operational information to the camera user. Since LCD displays do not generate light, it is common to use backlighting to make the LCD display visible under low ambient light conditions. EL panels are useful for this purpose, particularly in the case of cameras, due to their relatively low power consumption. A drawback in the use of EL backlighting relates to the requirement for an AC power source to activate the EL panel thereby necessitating a DC/AC converter of some sort to convert the available battery power to the AC drive needed to operate the EL backlight. In the case of a camera with a microprocessor controller, a 50% duty cycle switching output is available from the microprocessor that can be used to switch a DC/AC converter. This is not an optimum duty cycle for EL backlight power since, the higher the ON time for EL backlight, the higher the light output. While the duty cycle can be changed by reprogramming the microprocessor, it adds to the microprocessor""s program overhead and is thus costly to implement.
In accordance with the invention, an EL backlight drive circuit for a camera display is provided which comprises a flash circuit having a high voltage flash storage capacitor; an EL backlight device connected to the flash storage capacitor such that the flash storage capacitor serves as a DC power source for the EL backlight device; and a source of a pulse train having a fixed first duty cycle. The drive circuit also comprises a switching circuit including a duty cycle changing circuit and an electronic switching device coupled between said pulse train source and said EL backlight device and operative to convert DC voltage from said flash storage capacitor to pulsed voltage across said EL backlight device at a second duty cycle that is higher than the first duty cycle of the source pulse train. An advantage of the drive circuit of the invention is that a simple 50% duty pulse train from a camera microprocessor or ASIC can be employed as the pulse source to switch the DC voltage from the flash capacitor while achieving a significant improvement in EL backlight operating efficiency at a higher duty cycle rate, such as a 90% on time.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.